This invention relates to high power high voltage transistors, and is particularly directed to a transistor structurally and electrically configured for very high power, high efficiency amplification of radio frequency (RF) energy in the MF-HF-VHF RF spectrum. The invention is more particularly concerned with a transistor capable of providing output power of between one and several kilowatts in RF applications without problems of heat dissipation and with minimal complexity of circuitry. The invention is independent of any one particular transistor technology (BJT, FET, IGBT) or any particular class of operation (A, AB, B, C, D, E, or F).
In conventional HF and VHF high power RF amplifiers, the final power stage transistors are traditionally configured as RF common source (FET) or RF common emitter (BJT). The common source (or emitter) terminal is grounded with respect to both the input and output RF current paths, and therefore the opposite polarity DC electrode, namely the drain (or collector) terminal must be electrically isolated from ground. This requires either internal or external insulators. As a result, any associated heat developed in the transistor die must pass through an electrically insulating layer to reach the associated heat sink. This layer, being a dielectric, increases the capacitance between the drain and ground (or between the collector and ground). Optimally thin insulating layers are less of a barrier to heat transfer; therefore, very thin ceramic insulators are traditionally used, such as beryllium oxide (BeO), aluminum nitride (AlN), or thin diamond film. However, at substantially high power levels even these create an unacceptably large thermal gradient: this limits the available reliable power for a given transistor die. Also, the high dielectric constants exacerbate the drain-to-ground capacitance, limiting the performance at the higher RF frequencies.
Conventional HF high power RF amplifier stages are typically operated at moderately low voltages of 12 to 50 volts. Therefore, the associated high currents generate I-squared-R heating of the transistor and its interconnecting RF circuits, and their integral passive circuit elements. It has not been possible previously to reliably reduce this heating problem, simply by using higher voltages and lower currents. Conventional prior-art RF power generators employ multi-chip internally isolated transistor arrays, as shown in Table A, below. This necessitates operating at higher temperatures. Because the prior-art transistors employ ceramic insulators between the die and mounting flange, adequate thermal management to adjacent heat sinks continues to be a critical problem.